1. Field of the Invention
This invention relates to coating compositions which, when applied to substrate surfaces and exposed to actinic radiation, can form a crosslinked and cured film having excellent abrasion resistance, surface smoothness, thermal resistance, chemical resistance, durability, weather resistance and adhesion to the substrate.
2. Prior Art
Synthetic resin molded articles formed of polymethyl methacrylate resin, polymethacrylimide resin, polycarbonate resins, polystyrene resin and AS resin are not only light in weight and excellent in impact resistance, but also have high transparency. In recent years, therefore, they have come to be widely used as automobile plastic parts including headlamps, glazings, instrument covers and the like. On the other hand, the surfaces of such synthetic resin molded articles do not have sufficient abrasion resistance, so that they are subject to damage as a result of contact with other harder objects, friction, scratching and the like. Since any damage caused to the surfaces of such synthetic resin molded articles diminishes their commercial value significantly and/or makes them unusable in a short period of time, it is strongly desired to improve the abrasion resistance of their surfaces. Moreover, where they are used as automobile parts, their weather resistance is also regarded as essential.
In order to overcome the above-described disadvantage of synthetic resin molded articles, various attempts have heretofore been made. For example, U.S. Pat. No. 4,006,271 discloses a method for improving the abrasion resistance of synthetic resin molded articles wherein a coating material comprising a silicone or melamine resin composition is applied to the surface of a synthetic resin molded article and thermally condensed to form a crosslinked film. Although a high degree of abrasion resistance can be achieved by this method, the resulting film tends to have insufficient adhesion to the surface of the molded article. In order to enhance this adhesion, it is necessary to use an acrylic polymer or a silicone as a primer. This is disadvantageous in that a complicated treating procedure is required. Moreover, the curing time is so long that economic losses and low productivity results.
In order to overcome these disadvantages, there have been proposed a number of methods wherein a resin composition comprising radical-polymerizable monomers is applied to a substrate and then exposed to actinic radiation to form a crosslinked film (U.S. Pat. Nos. 3,518,341, 4,041,120, 4,291,097 and 4,367,245).
These methods make it possible to produce highly abrasion-resistant synthetic resin molded articles in a short time and with high productivity. However, in order to use them as outer trim plastic parts for automobiles and the like, their durability, chemical resistance and weather resistance are less than satisfactory.
Especially in the case of synthetic resin molded articles formed of materials having inherently poor weather resistance, such as polycarbonate resins, the substrate may be deteriorated by the action of the actinic radiation (e.g., ultraviolet light) having passed through the cured film formed thereon, granting that the cured film itself has good weather resistance. As a result, it may happen that the molded articles undergo a marked yellowing and/or the cured film formed on the surfaces thereof is cracked or separated.